Cardiac pacemakers, implantable cardioverter defibrillators (“ICDs”), and cardiac resynchronization therapy (“CRT”) devices are all implantable medical devices (“IMDs”). Pacemakers are designed to emit pacing stimuli to one or more chambers of the heart to treat bradyarrhythmia. ICDs typically have pacemaker functions and also include high voltage output capability that can be used to treat potentially lethal tachyarrhythmias. CRT devices provide pacing therapy to synchronize the left and right ventricles as a treatment for heart failure. CRT devices may or may not have high voltage defibrillation capability (CRT-P or CRT-D).
These devices are battery powered and, once implanted, require regular follow-up by a physician or health care professional to assess proper system operation and continued remaining battery capacity. The follow-ups typically occur at regular intervals, for example, every six months. Modern IMDs can communicate with an external computing device in a bi-directional fashion. This external computing device, known as a “programmer,” allows the physician or health care professional to retrieve various diagnostic data, review stored history about the patient and device operation, and change various parameters of the device. The programmer also can show information related to battery status.
Batteries for IMDs typically use lithium iodide (Lil), lithium silver vanadium pentoxide (SVO), or lithium carbon monoflouride (CFx) chemistries. Depending on the chemistry, battery depletion status can be assessed by the device by measuring battery voltage, the time required to charge internal capacitors, use of a coulomb charge counter, or some combination of these or other methods. Specific algorithms for determining battery status vary by manufacturer, chemistry, and individual device.
Overall longevity for IMDs is estimated during the product development cycle. Models of battery capacity, expected variations in circuit performance, and clinical use conditions are all taken into account in these models. From this modeling effort, longevity estimates are created for various assumed clinical use conditions. The Instructions for Use (IFU) that is included in the finished device packaging and labeling will contain these battery longevity estimates.
During the life of the device, the estimated battery longevity remaining may be determined from a simple calculation of the estimated total longevity minus the portion of life already consumed from the battery. Many IMDs currently marketed can make such calculations and display them to the user through a programmer.
The longevity of an IMD may vary widely depending upon clinical use conditions. The programmed amplitude of pacing pulses, for example, can affect CRT device longevity by a factor of two or more (i.e., half the typical lifetime), depending on the number of pacing pulses the CRT device emits. Many physicians and health care professionals are surprised at the impact these clinical use conditions and parameter settings have on device longevity and express displeasure when overall longevity varies significantly from typical values.
Thus, a need exists for systems and methods that can inform physicians or health care professionals of circumstances that are leading to sub-optimal (shorter than usual device longevity, and can allow the physicians or health care providers to understand (and perhaps modify) specific parameters that are causing the sub-optimal longevity.